1. Field of the Invention
The present invention relates to lanthanum boride sintered bodies and methods for producing the same.
2. Description of the Related Art
As proposed in the art (see, for example, Patent Literature 1), conventional lanthanum boride sintered bodies are produced by mixing lanthanum boride powders having an average particle diameter of 1 to 5 μm, a carbon content of less than 0.1 mass % and an oxygen content of less than 1.0 mass % with lanthanum boride powders having an average particle diameter of 50 to 500 nm, and sintering the mixture by hot pressing at a temperature of not less than 1800° C. and a pressure of not less than 20 MPa. Such sintered bodies allegedly have a high purity and exhibit an increased density, and a relative density of 93% has been obtained. It is also proposed that lanthanum boride green compacts are pressed in a hot press furnace and are thereafter subjected to an HIP treatment to further reduce the porosity; a relative density of 93% has been obtained (see, for example, Patent Literature 2). It is further proposed that a metal powder such as Fe, Co or Ni is admixed with a lanthanum boride powder having an average particle diameter of 1 to 20 μm, and the mixture is compacted, subjected to a pressure of 1 to 100 MPa and sintered at a temperature of about 1500 to 2000° C. in a spark plasma sintering furnace; a relative density of 99.3% has been obtained (see, for example, Patent Literature 3). It is furthermore proposed that a green compact of lanthanum boride and metallic silicon is sintered at a pressure in the range from 1.33×104 Pa to atmospheric pressure; a relative density of 91% has been obtained (see, for example, Patent Literature 4). Sintered bodies obtained by such a method allegedly exhibit higher stability and improved thermionic emission characteristics.    [PTL 1] Japanese Unexamined Patent Application Publication No. 2011-063487    [PTL 2] Japanese Unexamined Patent Application Publication No. 2011-195337    [PTL 3] Japanese Unexamined Patent Application Publication No. 2004-250725    [PTL 4] Japanese Unexamined Patent Application Publication No. 04-228474